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Overspeccing PV Question

  • 18-01-2023 7:51pm
    #1
    Registered Users Posts: 876 ✭✭✭


    Hi there,

    I was due to get a 6kW installation with a Solis Hybrid 6kW inverter.

    However I'm in the very fortunate position that I have a lot of roof space so when the installers came out, I asked what the max number of panels it could take. They said it could take 26x400W panels, which would make a 10.4kW installation and all south-facing. They actually said it could take more but that the inverter wouldn't be able to take it.

    I'm a bit new to this so would like some advice. Obviously overspeccing the panels compared to the inverter will be most helpful during winter months but is something of overkill during summer. So some questions if I'm going to do down this route.

    1) If it's a perfect day and it's producing 10kW, what happens to the 4kW difference between what the panels are generating and the inverter capacity? Can I sell that extra 4kW to the grid or send it to a battery or is it just completely lost potential?

    2) Should I just get a higher capacity inverter if I'm getting such a large system?



Comments

  • Registered Users Posts: 2,216 ✭✭✭SD_DRACULA


    1) you won't get any 4kw extra, inverter is limited to 6kw so that's the max it can output and on the 6kwh Solis 24 panels is max I'd say, 26 might be pushing it.

    2) Sunsynk do 8kw hybrid inverters but stock is impossible to find. Other option is to put 6kwp on the Solis hybrid and then get a second standard inverter and put the rest of the panels on that. In summer you might not be able to use it (unless you got an EV or two) but will help in winter for sure.



  • Registered Users Posts: 876 ✭✭✭eclipsechaser


    Thanks for that. So a second inverter is also a possiblity. Would, for instance, two 5kw inverters work just the same as a single 10kw inverter or what would the difference of having two of them make?

    We have an EV so I would like to produce as much as possible but don't want to be wasteful if 10kW of panels on a 6kW inverter is silly.



  • Moderators, Sports Moderators Posts: 18,617 Mod ✭✭✭✭slave1


    I’d question 26 400w panels into a 6kW inverter, what the model of the inverter and what is the model of the panels, it just sounds way too much especially if South facing



  • Moderators, Home & Garden Moderators Posts: 5,737 Mod ✭✭✭✭graememk


    The new 400+w longi has a lower VOC than the previous generation 375w ones.

    13 per string is the limit.

    There will be a lot of clipping in the summer -but the power just wouldn't be generated,



  • Registered Users Posts: 876 ✭✭✭eclipsechaser


    The electrician said 13 per roof was the maximum so that ties in with what you just said Graeme.

    The original contract (from half a year ago) was for 370W panels so it came as a surprise to me when they arrived with the 400W ones. I'll double check the model tomorrow.

    Can anyone explain how two inverters would work beside each other in practical terms?



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  • Registered Users Posts: 876 ✭✭✭eclipsechaser


    They're JAM53S31-400MR panels.

    VOC is 37.07V. VMP is 31.01V. ISC 13.79A. IMP 12.9A. Max overcurrent protection: 25A. Maximum system voltage 1500V.

    Will 26 of these be safe with a Solis Hybrid 6kW inverter?



  • Moderators, Home & Garden Moderators Posts: 5,737 Mod ✭✭✭✭graememk


    this calculation is for the longi. Taking in tolerances, Cold, temp coeff etc, If panel voltage goes too high the inverter will error out (I dont think it gets damaged... but I wouldnt want to push it.) it is 600V per string.

    Worst scenario at -20 degrees is 40.8 x 12 panel x 1.1215 temp coeff x 1.03 tolerance = 565.55V

    the temp coefficient may vary a bit between panels. but shouldnt change too much. (minus 20 is worst case scenario, I had worked out a -10 value but have lost it)

    putting your VOC in of 37.07

    37.07 x 13 panel x 1.1215 temp coeff x 1.03 tolerance = 603V

    Its right at the limit.. but that is at minus 20 remember.

    your panel is -0.275% for every degree the panel is above STC (25C, Panel temp) so for -10 it would be 35degree difference, which is 9.6% or 1.096 temp coeff that works out at 544V



  • Registered Users Posts: 5,558 ✭✭✭creedp


    Im clueless when it comes to interpreting data on electricity so just wondering if someone could give me a view on whether this data taken from the EC Commission model using an 8.4kwp solay array for my location would be safe to use with a 6kw hybrid inverter. Obviously fine for winter months but would the very high generation in the summer months be excessive for the inverter?



  • Moderators, Sports Moderators Posts: 18,617 Mod ✭✭✭✭slave1


    The post above yours explains it, get the maximum voltage per string from the Inverter spec sheet, then get the voltage from the panel spec sheet and apply to the formulae



  • Registered Users Posts: 4,873 ✭✭✭Padre_Pio


    My main question would be do you need it?

    There's a limit to how much you can feed back to the grid I think.



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  • Moderators, Sports Moderators Posts: 18,617 Mod ✭✭✭✭slave1


    No limit per se, totally depends on what papers you file with esbnetworks, most go with NC6 (free) which is 6kW but you can go above that with NC7(€1k) filing



  • Registered Users Posts: 5,558 ✭✭✭creedp


    My understanding is that the inverter imposes the limit on what generation can be fed back to the grid, and currently 6kw is the maximum that can be supplied, but I'm not clear if any excess generation can be self-consumed or diverted to batteries. I suppose the other consideration is that the level of generation is high for only 4 to 5 months (see table attached which I meant to attach to my earlier post) so the additional panels would be very beneficial for the remaining 7 months.




  • Registered Users Posts: 5,558 ✭✭✭creedp


    I have a quote for Tiger jinko 400 - 420 watt panels and a SOFAR 6KTLM-G2 inverter. According to spec sheet the MPPT Voltage range is 90 to 580 volts and the VoC of panels is 38.15v (STC). So the formula would give a max voltage per string for 10 panels of 38.15 x 10 x 1.1215 temp coeff x 1.03 tolerance = 441volts so within safe range. So am I correct in saying that as the inverter has 2 MPPT's, 20 panels would be fine for this inverter.



  • Registered Users Posts: 6,169 ✭✭✭crisco10



    Lol, I think my installers who came this week did your house before mine. When we were talking about fitting panels on the roof, they said something like "yesterday, we planned to do 14 and ended up do 26" ..

    Small world, good luck with the rest of he works



  • Registered Users Posts: 876 ✭✭✭eclipsechaser


    Haha. Yeah, that'll be them so as I doubt that happens too often. They seem like very decent guys.

    In fairness to them, we hadn't got a site survey in advance (I actually booked them before I bought the house!) but I kept emailing the head office asking them to please make sure that they brought extra panels with them as I was positive they could fit more than 14. I'd say we'd have made it into the mid-30's if it weren't for the already installed solar thermal and the inverter limitations.

    Best of luck with your own installation. Their electrician is coming to us this Wednesday by the way. The lads thought it might be a couple of weeks later so hopefully they're fast with you too.



  • Registered Users Posts: 6,169 ✭✭✭crisco10


    Thanks good to know about the sparks, they had said to me they were a week or two behind them, so be nice if it was more the former than the latter.



  • Moderators, Society & Culture Moderators Posts: 16,179 Mod ✭✭✭✭Manic Moran


    This. What are you going to do with the excess? What's the purpose of the install?

    Some folks want to be 100% independent or 100% green. That means you need to install sufficient panels to cover the electricity you use in your highest-consumption month. (Assuming you get to store in a battery or get full credit for excess generation). That also means that in your low-consumption months (assuming generation doesn't fall as much) you have excess power for which you may not get full 'dollar value.' ESB doesn't seem to pay full price for the power you export.

    I went for 'cost reduction'. I put on however many panels were needed for my lowest month of consumption, on the basis that 100% of the panels would be used at 100% capacity all the time. I have no problem with paying the price of the excess I use in other months, I was going to be paying it anyway. I just don't have to pay for the baseline power I'm generating. I have a lot more roof-space, I just don't see the merit in using it.



  • Moderators, Sports Moderators Posts: 18,617 Mod ✭✭✭✭slave1


    There is not a limit, this comes up more and more on the forum. If folk are filing their presence in the PV world then they 99.99% file a NC6 form (free) which limits instantaneous export to 6kW, file a NC7 (which costs money) and this can be increased.

    NC6 will suit 99% of folks getting a standard install



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